Overview of PL/SQL Units

PL/SQL is a modern, block-structured programming language. It provides several features that make developing powerful database applications very convenient. For example, PL/SQL provides procedural constructs, such as loops and conditional statements, that are not available in standard SQL.

PL/SQL code runs on the server, so using PL/SQL lets you centralize significant parts of your database applications for increased maintainability and security. It also enables you to achieve a significant reduction of network overhead in client/server applications.

Note:

Some Oracle tools, such as Oracle Forms, contain a PL/SQL engine that lets you run PL/SQL locally.

You can even use PL/SQL for some database applications in place of 3GL programs that use embedded SQL or Oracle Call Interface (OCI).

Anonymous Blocks

An anonymous block is a PL/SQL unit that has no name. An anonymous block consists of an optional declarative part, an executable part, and one or more optional exception handlers.

The declarative part declares PL/SQL variables, exceptions, and cursors. The executable part contains PL/SQL code and SQL statements, and can contain nested blocks.

Exception handlers contain code that is invoked when the exception is raised, either as a predefined PL/SQL exception (such as NO_DATA_FOUND or ZERO_DIVIDE) or as an exception that you define.

Anonymous blocks are usually used interactively from a tool, such as SQL*Plus, or in a precompiler, OCI, or SQL*Module application. They are usually used to invoke stored subprograms or to open cursor variables.

The anonymous block in Example 7-1 uses the DBMS_OUTPUT package to print the names of all employees in the HR.EMPLOYEES table who are in department 20.

Exceptions let you handle Oracle Database error conditions with PL/SQL program logic. This enables your application to prevent the server from issuing an error that can cause the client application to end. The anonymous block in Example 7-2 handles the predefined Oracle Database exception NO_DATA_FOUND (which results in ORA-01403 if not handled).

Naming Subprograms

Because a subprogram is stored in the database, it must be named. This distinguishes it from other stored subprograms and makes it possible for applications to invoke it. Each publicly-visible subprogram in a schema must have a unique name, and the name must be a legal PL/SQL identifier.

Note:

If you plan to invoke a stored subprogram using a stub generated by SQL*Module, then the stored subprogram name must also be a legal identifier in the invoking host 3GL language, such as Ada or C.

Subprogram Parameters

Stored subprograms can take parameters. In the procedure in Example 7-4, the department number is an input parameter that is used when the parameterized cursor c1 is opened.

The formal parameters of a subprogram have three major attributes, described in Table 7-1.

Table 7-1 Attributes of Subprogram Parameters

Parameter Attribute

Description

Name

This must be a legal PL/SQL identifier.

Mode

This indicates whether the parameter is an input-only parameter (IN), an output-only parameter (OUT), or is both an input and an output parameter (INOUT). If the mode is not specified, then IN is assumed.

Parameter Modes

Parameter modes define the action of formal parameters. You can use the three parameter modes, IN (the default), OUT, and INOUT, with any subprogram. Avoid using the OUT and INOUT modes with functions. Good programming practice dictates that a function returns a single value and does not change the values of variables that are not local to the subprogram.

Parameter Data Types

Numerically constrained types such as NUMBER(2) or VARCHAR2(20) are not allowed in a parameter list.

%TYPE and %ROWTYPE Attributes

Use the type attributes %TYPE and %ROWTYPE to constrain the parameter. For example, the procedure heading in Example 7-4 can be written as follows:

PROCEDURE get_emp_names(dept_num IN EMPLOYEES.DEPARTMENT_ID%TYPE)

This gives the dept_num parameter the same data type as the DEPARTMENT_ID column in the EMPLOYEES table. The column and table must be available when a declaration using %TYPE (or %ROWTYPE) is elaborated.

Using %TYPE is recommended, because if the type of the column in the table changes, it is not necessary to change the application code.

If the get_emp_names procedure is part of a package, you can use previously-declared public (package) variables to constrain its parameter data types. For example:

Use the %ROWTYPE attribute to create a record that contains all the columns of the specified table. The procedure in Example 7-5 returns all the columns of the EMPLOYEES table in a PL/SQL record for the given employee ID.

Tables and Records

You can pass PL/SQL tables as parameters to stored subprograms. You can also pass tables of records as parameters.

Note:

When passing a user defined type, such as a PL/SQL table or record to a remote subprogram, to make PL/SQL use the same definition so that the type checker can verify the source, you must create a redundant loop back DBLINK so that when the PL/SQL compiles, both sources pull from the same location.

Default Parameter Values

Parameters can take default values. Use the DEFAULT keyword or the assignment operator to give a parameter a default value. For example, the specification for the Get_emp_names procedure can be written as the following:

PROCEDURE Get_emp_names (Dept_num IN NUMBER DEFAULT 20) IS ...

or

PROCEDURE Get_emp_names (Dept_num IN NUMBER := 20) IS ...

When a parameter takes a default value, it can be omitted from the actual parameter list when you invoke the subprogram. When you do specify the parameter value on the invocation, it overrides the default value.

Note:

Unlike in an anonymous PL/SQL block, you do not use the keyword DECLARE before the declarations of variables, cursors, and exceptions in a stored subprogram. In fact, it is an error to use it.

Creating Subprograms

Use a text editor to write the subprogram. Then, using an interactive tool such as SQL*Plus, load the text file containing the procedure by entering the following statement:

SQL> @get_emp

This loads the procedure into the current schema from the get_emp.sql file (.sql is the default file extension). The slash (/) at the end of the code is not part of the code, it only activates the loading of the procedure.

Caution:

When developing a new subprogram, it is usually preferable to use the statement CREATEORREPLACEPROCEDURE or CREATEORREPLACEFUNCTION. This statement replaces any previous version of that subprogram in the same schema with the newer version, but without warning.

You can use either the keyword IS or AS after the subprogram parameter list.

To create a subprogram, a package specification, or a package body, you must meet the following prerequisites:

You must have the CREATEPROCEDURE system privilege to create a subprogram or package in your schema, or the CREATEANYPROCEDURE system privilege to create a subprogram or package in another user's schema. In either case, the package body must be created in the same schema as the package.

Note:

To create without errors (to compile the subprogram or package successfully) requires the following additional privileges:

The owner of the subprogram or package must be explicitly granted the necessary object privileges for all objects referenced within the body of the code.

The owner cannot obtain required privileges through roles.

If the privileges of the owner of a subprogram or package change, then the subprogram must be reauthenticated before it is run. If a necessary privilege to a referenced object is revoked from the owner of the subprogram or package, then the subprogram cannot be run.

The EXECUTE privilege on a subprogram gives a user the right to run a subprogram owned by another user. Privileged users run the subprogram under the security domain of the owner of the subprogram. Therefore, users need not be granted the privileges to the objects referenced by a subprogram. This allows for more disciplined and efficient security strategies with database applications and their users. Furthermore, all subprograms and packages are stored in the data dictionary (in the SYSTEM tablespace). No quota controls the amount of space available to a user who creates subprograms and packages.

Note:

Package creation requires a sort. The user creating the package must be able to create a sort segment in the temporary tablespace with which the user is associated.

Altering Subprograms

To alter a subprogram, you must first drop it using the DROPPROCEDURE or DROPFUNCTION statement, then re-create it using the CREATEPROCEDURE or CREATEFUNCTION statement. Alternatively, use the CREATEORREPLACEPROCEDURE or CREATEORREPLACEFUNCTION statement, which first drops the subprogram if it exists, then re-creates it as specified.

Caution:

The subprogram is dropped without warning.

Dropping Subprograms and Packages

A standalone subprogram, a standalone function, a package body, or an entire package can be dropped using the SQL statements DROPPROCEDURE, DROPFUNCTION, DROPPACKAGEBODY, and DROPPACKAGE, respectively. A DROPPACKAGE statement drops both the specification and body of a package.

The following statement drops the Old_sal_raise procedure in your schema:

DROP PROCEDURE Old_sal_raise;

Privileges Needed

To drop a subprogram or package, the subprogram or package must be in your schema, or you must have the DROPANYPROCEDURE privilege. An individual subprogram within a package cannot be dropped; the containing package specification and body must be re-created without the subprograms to be dropped.

External Subprograms

A PL/SQL subprogram executing on an Oracle Database instance can invoke an external subprogram written in a third-generation language (3GL). The 3GL subprogram runs in a separate address space from that of the database.

PL/SQL Function Result Cache

Using the PL/SQL function result cache can save significant space and time. Each time a result-cached PL/SQL function is invoked with different parameter values, those parameters and their result are stored in the cache. Subsequently, when the same function is invoked with the same parameter values, the result is retrieved from the cache, instead of being recomputed. Because the cache is stored in a shared global area (SGA), it is available to any session that runs your application.

If a database object that was used to compute a cached result is updated, the cached result becomes invalid and must be recomputed.

The best candidates for result-caching are functions that are invoked frequently but depend on information that changes infrequently or never.

The specification part of a package declares the public types, variables, constants, and subprograms that are visible outside the immediate scope of the package. The body of a package defines the objects declared in the specification, as well as private objects that are not visible to applications outside the package.

The SQL*Plus script in Example 7-6 creates a package that contains one stored function and two stored procedures, and then invokes one of the procedures.

PL/SQL Object Size Limits

The size limit for PL/SQL stored database objects such as subprograms, triggers, and packages is the size of the Descriptive Intermediate Attributed Notation for Ada (DIANA) code in the shared pool in bytes. The Linux and UNIX limit on the size of the flattened DIANA/code size is 64K but the limit might be 32K on desktop platforms.

The most closely related number that a user can access is the PARSED_SIZE in the static data dictionary view *_OBJECT_SIZE. That gives the size of the DIANA in bytes as stored in the SYS.IDL_xxx$ tables. This is not the size in the shared pool. The size of the DIANA part of PL/SQL code (used during compilation) is significantly larger in the shared pool than it is in the system table.

Creating Packages

Each part of a package is created with a different statement. Create the package specification using the CREATEPACKAGE statement. The CREATEPACKAGE statement declares public package objects.

To create a package body, use the CREATEPACKAGEBODY statement. The CREATEPACKAGEBODY statement defines the procedural code of the public subprograms declared in the package specification.

You can also define private, or local, package subprograms, and variables in a package body. These objects can only be accessed by other subprograms in the body of the same package. They are not visible to external users, regardless of the privileges they hold.

It is often more convenient to add the ORREPLACE clause in the CREATEPACKAGE or CREATEPACKAGEBODY statements when you are first developing your application. The effect of this option is to drop the package or the package body without warning. The CREATE statements are:

CREATE OR REPLACE PACKAGE Package_name AS ...

and

CREATE OR REPLACE PACKAGE BODY Package_name AS ...

Creating Packaged Objects

The body of a package can contain:

Subprograms declared in the package specification.

Definitions of cursors declared in the package specification.

Local subprograms, not declared in the package specification.

Local variables.

Subprograms, cursors, and variables that are declared in the package specification are global. They can be invoked, or used, by external users that have EXECUTE permission for the package or that have EXECUTEANYPROCEDURE privileges.

When you create the package body, ensure that each subprogram that you define in the body has the same parameters, by name, data type, and mode, as the declaration in the package specification. For functions in the package body, the parameters and the return type must agree in name and type.

Naming Packages and Package Objects

The names of a package and all public objects in the package must be unique within a given schema. The package specification and its body must have the same name. All package constructs must have unique names within the scope of the package, unless overloading of subprogram names is desired.

Package Invalidations and Session State

Each session that references a package object has its own instance of the corresponding package, including persistent state for any public and private variables, cursors, and constants. If any of the session's instantiated packages (specification or body) are invalidated, then all package instances in the session are invalidated and recompiled. As a result, the session state is lost for all package instances in the session.

When a package in a given session is invalidated, the session receives ORA-04068 the first time it attempts to use any object of the invalid package instance. The second time a session makes such a package call, the package is reinstantiated for the session without error.

Note:

For optimal performance, Oracle Database returns this error message only once—each time the package state is discarded.

If you handle this error in your application, ensure that your error handling strategy can accurately handle this error. For example, when a subprogram in one package invokes a subprogram in another package, your application must be aware that the session state is lost for both packages.

In most production environments, DDL operations that can cause invalidations are usually performed during inactive working hours; therefore, this situation might not be a problem for end-user applications. However, if package invalidations are common in your system during working hours, then you might want to code your applications to handle this error when package calls are made.

Packages Supplied with Oracle Database

There are many packages provided with Oracle Database, either to extend the functionality of the database or to give PL/SQL access to SQL features. You can invoke these packages from your application.

Overview of Bulk Binding

Oracle Database uses two engines to run PL/SQL blocks and subprograms. The PL/SQL engine runs procedural statements, while the SQL engine runs SQL statements. During execution, every SQL statement causes a context switch between the two engines, resulting in performance overhead.

Performance can be improved substantially by minimizing the number of context switches required to run a particular block or subprogram. When a SQL statement runs inside a loop that uses collection elements as bind variables, the large number of context switches required by the block can cause poor performance. Collections include the following:

Varrays

Nested tables

Index-by tables

Host arrays

Binding is the assignment of values to PL/SQL variables in SQL statements. Bulk binding is binding an entire collection at once. Bulk binds pass the entire collection back and forth between the two engines in a single operation.

Typically, using bulk binds improves performance for SQL statements that affect four or more database rows. The more rows affected by a SQL statement, the greater the performance gain from bulk binds.

Note:

This section provides an overview of bulk binds to help you decide whether to use them in your PL/SQL applications. For detailed information about using bulk binds, including ways to handle exceptions that occur in the middle of a bulk bind operation, see Oracle Database PL/SQL Language Reference.

Parallel DML is disabled with bulk binds.

When to Use Bulk Binds

Consider using bulk binds to improve the performance of the following:

DML Statements that Reference Collections

A bulk bind, which uses the FORALL keyword, can improve the performance of INSERT, UPDATE, or DELETE statements that reference collection elements.

The PL/SQL block in Example 7-7 increases the salary for employees whose manager's ID number is 7902, 7698, or 7839, with and without bulk binds. Without bulk bind, PL/SQL sends a SQL statement to the SQL engine for each updated employee, leading to context switches that slow performance.

SELECT Statements that Reference Collections

The BULKCOLLECTINTO clause can improve the performance of queries that reference collections. You can use BULKCOLLECTINTO with tables of scalar values, or tables of %TYPE values.

The PL/SQL block in Example 7-8 queries multiple values into PL/SQL tables, with and without bulk binds. Without bulk bind, PL/SQL sends a SQL statement to the SQL engine for each selected employee, leading to context switches that slow performance.

FOR Loops that Reference Collections and Return DML

You can use the FORALL keyword with the BULKCOLLECTINTO keywords to improve the performance of FOR loops that reference collections and return DML.

The PL/SQL block in Example 7-9 updates the EMPLOYEES table by computing bonuses for a collection of employees. Then it returns the bonuses in a column called bonus_list_inst. The actions are performed with and without bulk binds. Without bulk bind, PL/SQL sends a SQL statement to the SQL engine for each updated employee, leading to context switches that slow performance.

Triggers

A trigger is a special kind of PL/SQL anonymous block. You can define triggers to fire before or after SQL statements, either on a statement level or for each row that is affected. You can also define INSTEADOF triggers or system triggers (triggers on DATABASE and SCHEMA).

Compiling PL/SQL Subprograms for Native Execution

You can speed up PL/SQL subprograms by compiling them into native code residing in shared libraries.

You can use native compilation with both the supplied packages and the subprograms you write yourself. Subprograms compiled this way work in all server environments, such as the shared server configuration (formerly known as multithreaded server) and Oracle Real Application Clusters (Oracle RAC).

This technique is most effective for computation-intensive subprograms that do not spend much time executing SQL, because it can do little to speed up SQL statements invoked from these subprograms.

With Java, you can use the ncomp tool to compile your own packages and classes.

Cursor Variables

A cursor is a static object; a cursor variable is a pointer to a cursor. Because cursor variables are pointers, they can be passed and returned as parameters to subprograms. A cursor variable can also refer to different cursors in its lifetime.

Additional advantages of cursor variables include the following:

Encapsulation

Queries are centralized in the stored subprogram that opens the cursor variable.

Easy maintenance

If you must change the cursor, then you only make the change in the stored subprogram, not in each application.

Convenient security

The user of the application is the username used when the application connects to the server. The user must have EXECUTE permission on the stored subprogram that opens the cursor. But, the user need not have READ permission on the tables used in the query. This capability can be used to limit access to the columns in the table, as well as access to other stored subprograms.

Declaring and Opening Cursor Variables

Memory is usually allocated for a cursor variable in the client application using the appropriate ALLOCATE statement. In Pro*C, use the EXECSQLALLOCATEcursor_name statement. In OCI, use the Cursor Data Area.

You can also use cursor variables in applications that run entirely in a single server session. You can declare cursor variables in PL/SQL subprograms, open them, and use them as parameters for other PL/SQL subprograms.

Examples of Cursor Variables

This section has the following examples of cursor variable usage in PL/SQL:

Example 7-10 creates a package that defines a PL/SQL cursor variable type and two procedures, and then invokes the procedures from a PL/SQL block. The first procedure opens a cursor variable using a bind variable in the WHERE clause. The second procedure uses a cursor variable to fetch rows from the EMPLOYEES table.

In Example 7-11, the procedure opens a cursor variable for either the EMPLOYEES table or the DEPARTMENTS table, depending on the value of the parameter discrim. The anonymous block invokes the procedure to open the cursor variable for the EMPLOYEES table, but fetches from the DEPARTMENTS table, which raises the predefined exception ROWTYPE_MISMATCH.

Handling PL/SQL Compile-Time Errors

To list compile-time errors, query the static data dictionary view *_ERRORS. From these views, you can retrieve original source code. The error text associated with the compilation of a subprogram is updated when the subprogram is replaced, and it is deleted when the subprogram is dropped.

SQL*Plus issues a warning message for compile-time errors, but for more information about them, you must use the command SHOWERRORS.

Note:

Before issuing the SHOWERRORS statement, use the SETLINESIZE statement to get long lines on output. The value 132 is usually a good choice. For example:

SET LINESIZE 132

Example 7-12 has two compile-time errors: WHER should be WHERE, and END should be followed by a semicolon. SHOWERRORS shows the line, column, and description of each error.

Handling Run-Time PL/SQL Errors

Oracle Database allows user-defined errors in PL/SQL code to be handled so that user-specified error numbers and messages are returned to the client application, which can handle the error.

User-specified error messages are returned using the RAISE_APPLICATION_ERROR procedure. For example:

RAISE_APPLICATION_ERROR(error_number, 'text', keep_error_stack)

This procedure stops subprogram execution, rolls back any effects of the subprogram, and returns a user-specified error number and message (unless the error is trapped by an exception handler). error_number must be in the range of -20000 to -20999.

Use error number -20000 as a generic number for messages where it is important to relay information to the user, but having a unique error number is not required. Text must be a character expression, 2 Kbytes or less (longer messages are ignored). Keep_error_stack can be TRUE if you want to add the error to any already on the stack, or FALSE if you want to replace the existing errors. By default, this option is FALSE.

Note:

Some of the Oracle Database packages, such as DBMS_OUTPUT, DBMS_DESCRIBE, and DBMS_ALERT, use application error numbers in the range -20000 to -20005. See the descriptions of these packages for more information.

The RAISE_APPLICATION_ERROR procedure is often used in exception handlers or in the logic of PL/SQL code. For example, the following exception handler selects the string for the associated user-defined error message and invokes the RAISE_APPLICATION_ERROR procedure:

Declaring Exceptions and Exception Handling Routines

User-defined exceptions are explicitly defined and signaled within the PL/SQL block to control processing of errors specific to the application. When an exception is raised (signaled), the usual execution of the PL/SQL block stops, and a routine called an exception handler is invoked. Specific exception handlers can be written to handle any internal or user-defined exception.

Application code can check for a condition that requires special attention using an IF statement. If there is an error condition, then two options are available:

Enter a RAISE statement that names the appropriate exception. A RAISE statement stops the execution of the subprogram, and control passes to an exception handler (if any).

Invoke the RAISE_APPLICATION_ERROR procedure to return a user-specified error number and message.

You can also define an exception handler to handle user-specified error messages. For example, Figure 7-1 shows the following:

An exception and associated exception handler in a subprogram

A conditional statement that checks for an error (such as transferring funds not available) and enters a user-specified error number and message within a trigger

How user-specified error numbers are returned to the invoking environment (in this case, a subprogram), and how that application can define an exception that corresponds to the user-specified error number

Declare a user-defined exception in a subprogram or package body (private exceptions), or in the specification of a package (public exceptions). Define an exception handler in the body of a subprogram (standalone or package).

Unhandled Exceptions

In database PL/SQL units, an unhandled user-error condition or internal error condition that is not trapped by an appropriate exception handler causes the implicit rollback of the program unit. If the program unit includes a COMMIT statement before the point at which the unhandled exception is observed, then the implicit rollback of the program unit can only be completed back to the previous COMMIT.

Additionally, unhandled exceptions in database-stored PL/SQL units propagate back to client-side applications that invoke the containing program unit. In such an application, only the application program unit invocation is rolled back (not the entire application program unit), because it is submitted to the database as a SQL statement.

If unhandled exceptions in database PL/SQL units are propagated back to database applications, modify the database PL/SQL code to handle the exceptions. Your application can also trap for unhandled exceptions when invoking database program units and handle such errors appropriately.

Handling Errors in Distributed Queries

You can use a trigger or a stored subprogram to create a distributed query. This distributed query is decomposed by the local Oracle Database instance into a corresponding number of remote queries, which are sent to the remote nodes for execution. The remote nodes run the queries and send the results back to the local node. The local node then performs any necessary post-processing and returns the results to the user or application.

If a portion of a distributed statement fails, possibly due to a constraint violation, then Oracle Database returns ORA-02055. Subsequent statements, or subprogram invocations, return ORA-02067 until a rollback or a rollback to savepoint is entered.

Design your application to check for any returned error messages that indicates that a portion of the distributed update has failed. If you detect a failure, rollback the entire transaction (or rollback to a savepoint) before allowing the application to proceed.

Handling Errors in Remote Subprograms

When a subprogram is run locally or at a remote location, the following types of exceptions can occur:

PL/SQL user-defined exceptions, which must be declared using the keyword EXCEPTION

PL/SQL predefined exceptions, such as NO_DATA_FOUND

SQL errors, such as ORA-00900

Application exceptions, which are generated using the RAISE_APPLICATION_ERROR procedure.

When using local subprograms, all of these messages can be trapped by writing an exception handler, such as shown in the following example:

EXCEPTION
WHEN ZERO_DIVIDE THEN
/* Handle the exception */

Notice that the WHEN clause requires an exception name. If the exception that is raised does not have a name, such as those generated with RAISE_APPLICATION_ERROR, then one can be assigned using PRAGMA_EXCEPTION_INIT, as shown in the following example:

When invoking a remote subprogram, exceptions are also handled by creating a local exception handler. The remote subprogram must return an error number to the local invoking subprogram, which then handles the exception, as shown in the previous example. Because PL/SQL user-defined exceptions always return ORA-06510 to the local subprogram, these exceptions cannot be handled. All other remote exceptions can be handled in the same manner as local exceptions.

Debugging Stored Subprograms

Compiling a stored subprogram involves fixing any syntax errors in the code. You might need to do additional debugging to ensure that the subprogram works correctly, performs well, and recovers from errors. Such debugging might involve:

PL/Scope

PL/Scope is a compiler-driven tool that collects and organizes data about user-defined identifiers from PL/SQL source code. Because PL/Scope is a compiler-driven tool, you use it through interactive development environments (such as SQL Developer and JDeveloper), rather than directly.

PL/Scope enables the development of powerful and effective PL/Scope source code browsers that increase PL/SQL developer productivity by minimizing time spent browsing and understanding source code.

PL/SQL Hierarchical Profiler

The PL/SQL hierarchical profiler reports the dynamic execution profile of your PL/SQL program, organized by subprogram calls. It accounts for SQL and PL/SQL execution times separately. Each subprogram-level summary in the dynamic execution profile includes information such as number of calls to the subprogram, time spent in the subprogram itself, time spent in the subprogram's subtree (that is, in its descendent subprograms), and detailed parent-children information.

You can browse the generated HTML reports in any browser. The browser's navigational capabilities, combined with well chosen links, provide a powerful way to analyze performance of large applications, improve application performance, and lower development costs.

Oracle JDeveloper

Recent releases of Oracle JDeveloper have extensive features for debugging PL/SQL, Java, and multi-language programs. You can get Oracle JDeveloper as part of various Oracle product suites. Often, a more recent release is available as a download at http://www.oracle.com/technology/.

DBMS_OUTPUT Package

You can also debug stored subprograms and triggers using the Oracle package DBMS_OUTPUT. Put PUT and PUT_LINE statements in your code to output the value of variables and expressions to your terminal.

Privileges for Debugging PL/SQL and Java Stored Subprograms

Starting with Oracle Database 10g, a new privilege model applies to debugging PL/SQL and Java code running within the database. This model applies whether you are using Oracle JDeveloper, Oracle Developer, or any of the various third-party PL/SQL or Java development environments, and it affects both the DBMS_DEBUG and DBMS_DEBUG_JDWP APIs.

For a session to connect to a debugger, the effective user at the time of the connect operation must have the DEBUGCONNECTSESSION system privilege. This effective user might be the owner of a DR routine involved in making the connect call.

When a debugger becomes connected to a session, the session login user and the currently enabled session-level roles are fixed as the privilege environment for that debugging connection. Any DEBUG or EXECUTE privileges needed for debugging must be granted to that combination of user and roles.

To be able to display and change Java public variables or variables declared in a PL/SQL package specification, the debugging connection must be granted either EXECUTE or DEBUG privilege on the relevant code.

To be able to either display and change private variables or breakpoint and execute code lines step by step, the debugging connection must be granted DEBUG privilege on the relevant code

Caution:

The DEBUG privilege allows a debugging session to do anything that the subprogram being debugged could have done if that action had been included in its code.

In addition to these privilege requirements, the ability to stop on individual code lines and debugger access to variables are allowed only in code compiled with debug information generated. Use the PL/SQL compilation parameter PLSQL_DEBUG and the DEBUG keyword on statements such as ALTERPACKAGE to control whether the PL/SQL compiler includes debug information in its results. If not, variables are not accessible, and neither stepping nor breakpoints stop on code lines. The PL/SQL compiler never generates debug information for code hidden with the PL/SQL wrap utility.

The DEBUGANYPROCEDURE system privilege is equivalent to the DEBUG privilege granted on all objects in the database. Objects owned by SYS are included if the value of the O7_DICTIONARY_ACCESSIBILITY parameter is TRUE.

A debug role mechanism is available to carry privileges needed for debugging that are not normally enabled in the session. See the documentation on the DBMS_DEBUG and DBMS_DEBUG_JDWP packages for details on how to specify a debug role and any necessary related password.

The JAVADEBUGPRIV role carries the DEBUGCONNECTSESSION and DEBUGANYPROCEDURE privileges. Grant it only with the care those privileges warrant.

Caution:

Granting DEBUGANYPROCEDURE privilege, or granting DEBUG privilege on any object owned by SYS, means granting complete rights to the database.

Writing Low-Level Debugging Code

If you are writing code for part of a debugger, you might need to use packages such as DBMS_DEBUG_JDWP or DBMS_DEBUG.

DBMS_DEBUG_JDWP Package

The DBMS_DEBUG_JDWP package, provided starting with Oracle9i Release 2, provides a framework for multi-language debugging that is expected to supersede the DBMS_DEBUG package over time. It is especially useful for programs that combine PL/SQL and Java.

DBMS_DEBUG Package

The DBMS_DEBUG package, provided starting with Oracle8i, implements server-side debuggers and provides a way to debug server-side PL/SQL units. Several of the debuggers available, such as Oracle Procedure Builder and various third-party vendor solutions, use this API.

You must have the EXECUTE privilege for the standalone subprogram or for the package containing the subprogram, or you must have the EXECUTEANYPROCEDURE system privilege.

If you are executing a remote subprogram, then you must be granted the EXECUTE privilege or EXECUTEANYPROCEDURE system privilege directly, not through a role.

You must include the name of the owner in the invocation. For example:

EXECUTE jdoe.Fire_emp (1043);
EXECUTE jdoe.Hire_fire.Fire_emp (1043);

If the subprogram is a definer's-rights (DR) subprogram, then it runs with the privileges of the owner. The owner must have all the necessary object privileges for any referenced objects.

If the subprogram is an invoker's-rights (IR) subprogram, then it runs with your privileges. You must have all the necessary object privileges for any referenced objects; that is, all objects accessed by the subprogram through external references that are resolved in your schema. You can hold these privileges either directly or through a role. Roles are enabled unless an IR subprogram is invoked directly or indirectly by a DR subprogram.

Invoking a Subprogram Interactively from Oracle Tools

You can invoke a subprogram interactively from an Oracle Database tool, such as SQL*Plus. Example 7-13 uses SQL*Plus to create a procedure and then invokes it in two different ways.

To run a subprogram within the code of a precompiler application, you must use the EXEC call interface. For example, the following statement invokes the Fire_emp procedure in the code of a precompiler application:

EXEC SQL EXECUTE
BEGIN
Fire_emp1(:Empnum);
END;
END-EXEC;

See Also:

For information about invoking PL/SQL subprograms from within 3GL applications:

Invoking Remote Subprograms

Remote subprograms (standalone and packaged) can be invoked from within a subprogram, OCI application, or precompiler by specifying the remote subprogram name, a database link, and the parameters for the remote subprogram.

For example, the following SQL*Plus statement invokes the procedure fire_emp1, which is located in the database and referenced by the local database link named boston_server:

EXECUTE fire_emp1@boston_server(1043);

You must specify values for all remote subprogram parameters, even if there are defaults. You cannot access remote package variables and constants.

Caution:

Remote subprogram invocations use run-time binding. The user account to which you connect depends on the database link. (Stored subprograms use compile-time binding.)

If a local subprogram invokes a remote subprogram, and a timestamp mismatch is found during execution of the local subprogram, then the remote subprogram is not run, and the local subprogram is invalidated.

The synonym enables you to invoke the remote subprogram from an Oracle Database tool application, such as a SQL*Forms application, as well from within a subprogram, OCI application, or precompiler.

Synonyms provide both data independence and location transparency. Synonyms permit applications to function without modification regardless of which user owns the object and regardless of which database holds the object. However, synonyms are not a substitute for privileges on database objects. Appropriate privileges must be granted to a user before the user can use the synonym.

Because subprograms defined within a package are not individual objects (the package is the object), synonyms cannot be created for individual subprograms within a package.

If you do not want to use a synonym, you can create a local subprogram to invoke the remote subprogram. For example:

Committing Transactions

All invocations to remotely stored subprograms are assumed to perform updates; therefore, this type of referencing always requires two-phase commit of that transaction (even if the remote subprogram is read-only). Furthermore, if a transaction that includes a remote subprogram invocation is rolled back, then the work done by the remote subprogram is also rolled back.

A subprogram invoked remotely can usually execute a COMMIT, ROLLBACK, or SAVEPOINT statement, the same as a local subprogram. However, there are some differences in action:

If the transaction was originated by a database that is not an Oracle database, as might be the case in XA applications, these operations are not allowed in the remote subprogram.

After doing one of these operations, the remote subprogram cannot start any distributed transactions of its own.

If the remote subprogram does not commit or roll back its work, the commit is done implicitly when the database link is closed. In the meantime, further invocations to the remote subprogram are not allowed because it is still considered to be performing a transaction.

A distributed update modifies data on two or more databases. A distributed update is possible using a subprogram that includes two or more remote updates that access data on different databases. Statements in the construct are sent to the remote databases, and the execution of the construct succeeds or fails as a unit. If part of a distributed update fails and part succeeds, then a rollback (of the entire transaction or to a savepoint) is required to proceed. Consider this when creating subprograms that perform distributed updates.

Invoking Stored PL/SQL Functions from SQL Statements

Caution:

Because SQL is a declarative language, rather than an imperative (or procedural) one, you cannot know how many times a function invoked from a SQL statement will execute—even if the function is written in PL/SQL, an imperative language.

If your application requires that a function be executed a certain number of times, do not invoke that function from a SQL statement. Use a cursor instead.

For example, if your application requires that a function be called once for each selected row, then open a cursor, select rows from the cursor, and call the function for each row. This guarantees that the number of calls to the function is the same as the number of rows fetched from the cursor.

To be invoked from a SQL statement, a stored PL/SQL function must be declared either at schema level or in a package specification.

The following SQL statements can invoke stored PL/SQL functions:

INSERT

UPDATE

DELETE

SELECT

CALL

(CALL can also invoke a stored PL/SQL procedure.)

To invoke a PL/SQL subprogram from SQL, you must either own or have EXECUTE privileges on the subprogram. To select from a view defined with a PL/SQL function, you must have SELECT privileges on the view. No separate EXECUTE privileges are necessary to select from the view.

If the query is parallelized, then SQL statements in your PL/SQL subprogram might also be run in parallel (using the parallel query option).

Where PL/SQL Functions Can Appear in SQL Statements

A PL/SQL function can appear in a SQL statement wherever a built-in SQL function or an expression can appear in a SQL statement. For example, a PL/SQL function can appear in the following:

Select list of the SELECT statement

Condition of the WHERE or HAVING clause

CONNECTBY, STARTWITH, ORDERBY, or GROUPBY clause

VALUES clause of the INSERT statement

SET clause of the UPDATE statement

A PL/SQL table function (which returns a collection of rows) can appear in a SELECT statement in place of the following:

Column name in the SELECT list

Table name in the FROM clause

A PL/SQL function cannot appear in the following contexts, which require unchanging definitions:

CHECK constraint clause of a CREATE or ALTERTABLE statement

Default value specification for a column

When PL/SQL Functions Can Appear in SQL Expressions

To be invoked from a SQL expression, a PL/SQL function must satisfy the following requirements:

It must be a row function, not a column (group) function; that is, its argument cannot be an entire column.

Its formal parameters must be IN parameters, not OUT or INOUT parameters.

Its formal parameters and its return value (if any) must have Oracle built-in data types (such as CHAR, DATE, or NUMBER), not PL/SQL data types (such as BOOLEAN, RECORD, or TABLE).

There is an exception to this rule: A formal parameter can have a PL/SQL data type if the corresponding actual parameter is implicitly converted to the data type of the formal parameter (as in Example 7-17).

In the SQL*Plus script in Example 7-17, the SQL statement CALL invokes the PL/SQL function f1, whose formal parameter and return value have PL/SQL data type PLS_INTEGER. The CALL statement succeeds because the actual parameter, 2, is implicitly converted to the data type PLS_INTEGER. If the actual parameter had a value outside the range of PLS_INTEGER, the CALL statement would fail.

Controlling Side Effects

The purity of a stored subprogram refers to the side effects of that subprogram on database tables or package variables. Side effects can prevent the parallelization of a query, yield order-dependent (and therefore, indeterminate) results, or require that package state be maintained across user sessions. Various side effects are not allowed when a function is invoked from a SQL query or DML statement.

In releases prior to Oracle8i, Oracle Database leveraged the PL/SQL compiler to enforce restrictions during the compilation of a stored subprogram or a SQL statement. Starting with Oracle8i, the compile-time restrictions were relaxed, and a smaller set of restrictions are enforced during execution.

This change provides uniform support for stored subprograms written in PL/SQL, Java, and C, and it allows programmers the most flexibility possible.

Restrictions

When a SQL statement is run, checks are made to see if it is logically embedded within the execution of an already running SQL statement. This occurs if the statement is run from a trigger or from a subprogram that was in turn invoked from the already running SQL statement. In these cases, further checks occur to determine if the new SQL statement is safe in the specific context.

The following restrictions are enforced on subprograms:

A subprogram invoked from a query or DML statement might not end the current transaction, create or rollback to a savepoint, or ALTER the system or session.

A subprogram invoked from a query (SELECT) statement or from a parallelized DML statement might not execute a DML statement or otherwise modify the database.

A subprogram invoked from a DML statement might not read or modify the particular table being modified by that DML statement.

These restrictions apply regardless of what mechanism is used to run the SQL statement inside the subprogram or trigger. For example:

They apply to a SQL statement invoked from PL/SQL, whether embedded directly in a subprogram or trigger body, run using the native dynamic mechanism (EXECUTEIMMEDIATE), or run using the DBMS_SQL package.

They apply to statements embedded in Java with SQLJ syntax or run using JDBC.

They apply to statements run with OCI using the callback context from within an "external" C function.

You can avoid these restrictions if the execution of the new SQL statement is not logically embedded in the context of the already running statement. PL/SQL's autonomous transactions provide one escape (see Autonomous Transactions ). Another escape is available using Oracle Call Interface (OCI) from an external C function, if you create a new connection rather than using the handle available from the OCIExtProcContext argument.

Declaring a Function

You can use the keywords DETERMINISTIC and PARALLEL_ENABLE in the syntax for declaring a function. These are optimization hints that inform the query optimizer and other software components about the following:

Functions that need not be invoked redundantly

Functions permitted within a parallelized query or parallelized DML statement

Only functions that are DETERMINISTIC are allowed in function-based indexes and in certain snapshots and materialized views.

A deterministic function depends solely on the values passed into it as arguments and does not reference or modify the contents of package variables or the database or have other side-effects. Such a function produces the same result value for any combination of argument values passed into it.

You place the DETERMINISTIC keyword after the return value type in a declaration of the function. For example:

Do not repeat the keyword on the function or method body in a CREATEPACKAGEBODY or CREATETYPEBODY statement.

Certain performance optimizations occur on invocations of functions that are marked DETERMINISTIC without any other action being required. The following features require that any function used with them be declared DETERMINISTIC:

Any user-defined function used in a function-based index.

Any function used in a materialized view, if that view is to qualify for Fast Refresh or is marked ENABLEQUERYREWRITE.

The preceding functions features attempt to use previously calculated results rather than invoking the function when it is possible to do so.

It is good programming practice to make functions that fall in the following categories DETERMINISTIC:

Functions used in a WHERE, ORDERBY, or GROUPBY clause

Functions that MAP or ORDER methods of a SQL type

Functions that help determine whether or where a row appears in a result set

Keep the following points in mind when you create DETERMINISTIC functions:

The database cannot recognize if the action of the function is indeed deterministic. If the DETERMINISTIC keyword is applied to a function whose action is not truly deterministic, then the result of queries involving that function is unpredictable.

If you change the semantics of a DETERMINISTIC function and recompile it, then existing function-based indexes and materialized views report results for the prior version of the function. Thus, if you change the semantics of a function, you must manually rebuild any dependent function-based indexes and materialized views.

Parallel Query and Parallel DML

Oracle Database's parallel execution feature divides the work of executing a SQL statement across multiple processes. Functions invoked from a SQL statement that is run in parallel might have a separate copy run in each of these processes, with each copy invoked for only the subset of rows that are handled by that process.

Each process has its own copy of package variables. When parallel execution begins, these are initialized based on the information in the package specification and body as if a new user is logging into the system; the values in package variables are not copied from the original login session. And changes made to package variables are not automatically propagated between the various sessions or back to the original session. Java STATIC class attributes are similarly initialized and modified independently in each process. Because a function can use package (or Java STATIC) variables to accumulate some value across the various rows it encounters, Oracle Database cannot assume that it is safe to parallelize the execution of all user-defined functions.

For SELECT statements in Oracle Database versions prior to 8.1.5, the parallel query optimization allowed functions noted as both RNPS and WNPS in a PRAGMARESTRICT_REFERENCES declaration to run in parallel. Functions defined with CREATEFUNCTION statements had their code implicitly examined to determine if they were pure enough; parallelized execution might occur even though a pragma cannot be specified on these functions.

For DML statements in Oracle Database versions prior to 8.1.5, the parallelization optimization looked to see if a function was noted as having all four of RNDS, WNDS, RNPS and WNPS specified in a PRAGMARESTRICT_REFERENCES declaration; those functions that were marked as neither reading nor writing to either the database or package variables could run in parallel. Again, those functions defined with a CREATEFUNCTION statement had their code implicitly examined to determine if they were actually pure enough; parallelized execution might occur even though a pragma cannot be specified on these functions.

Oracle Database versions 8.1.5 and later continue to parallelize those functions that earlier versions recognize as parallelizable. The PARALLEL_ENABLE keyword is the preferred way to mark your code as safe for parallel execution. This keyword is syntactically similar to DETERMINISTIC as described in Declaring a Function; it is placed after the return value type in a declaration of the function, as in:

A PL/SQL function defined with CREATEFUNCTION might still be run in parallel without any explicit declaration that it is safe to do so, if the system can determine that it neither reads nor writes package variables nor invokes any function that might do so. A Java method or C function is never seen by the system as safe to run in parallel, unless the programmer explicitly indicates PARALLEL_ENABLE on the call specification, or provides a PRAGMARESTRICT_REFERENCES indicating that the function is sufficiently pure.

An additional run-time restriction is imposed on functions run in parallel as part of a parallelized DML statement. Such a function is not permitted to in turn execute a DML statement; it is subject to the same restrictions that are enforced on functions that are run inside a query (SELECT) statement.

PRAGMA RESTRICT_REFERENCES for Backward Compatibility

In Oracle Database versions prior to 8.1.5 (Oracle8i), programmers used PRAGMARESTRICT_REFERENCES to assert the purity level of a subprogram. In subsequent versions, use the hints PARALLEL_ENABLE and DETERMINISTIC, instead, to communicate subprogram purity to Oracle Database.

You can remove PRAGMARESTRICT_REFERENCES from your code. However, this pragma remains available for backward compatibility in situations where one of the following is true:

It is impossible or impractical to edit existing code to remove PRAGMARESTRICT_REFERENCES completely. If you do not remove it from a subprogram S1 that depends on another subprogram S2, then PRAGMARESTRICT_REFERENCES might also be needed in S2, so that S1 will compile.

Replacing PRAGMARESTRICT_REFERENCES in existing code with hints PARALLEL_ENABLE and DETERMINISTIC would negatively affect the action of new, dependent code. Use PRAGMARESTRICT_REFERENCES to preserve the action of the existing code.

An existing PL/SQL application can thus continue using the pragma even on new functionality, to ease integration with the existing code. Do not use the pragma in a new application.

If you use PRAGMARESTRICT_REFERENCES, place it in a package specification, not in a package body. It must follow the declaration of a subprogram, but it need not follow immediately. Only one pragma can reference a given subprogram declaration.

The subprogram writes no database state (does not modify database tables).

RNDS

The subprogram reads no database state (does not query database tables).

WNPS

The subprogram writes no package state (does not change the values of packaged variables).

RNPS

The subprogram reads no package state (does not reference the values of packaged variables).

TRUST

The other restrictions listed in the pragma are not enforced; they are simply assumed to be true. This allows easy invocation from functions that have RESTRICT_REFERENCES declarations to those that do not.

You can pass the arguments in any order. If any SQL statement inside the subprogram body violates a rule, then you get an error when the statement is parsed.

In Example 7-18, the function compound neither reads nor writes database or package state; therefore, you can assert the maximum purity level. Always assert the highest purity level that a subprogram allows, so that the PL/SQL compiler never rejects the subprogram unnecessarily.

Using the Keyword TRUST

When PRAGMARESTRICTREFERENCES includes the keyword TRUST, the restrictions listed in the pragma are assumed to be true, and not enforced.

When you invoke a routine that is in a section of code that does not use pragmas (such as a Java method), from a section of PL/SQL code that does use pragmas, specify PRAGMARESTRICTREFERENCES with TRUST for either the invoked routine or the invoking routine.

In both Example 7-19 and Example 7-20, the PL/SQL function f invokes the Java procedure java_sleep. In Example 7-19, this is possible because java_sleep is declared to be WNDS with TRUST. In Example 7-20, it is possible because f is declared to be WNDS with TRUST, which allows it to invoke any routine.

Differences between Static and Dynamic SQL Statements

Static INSERT, UPDATE, and DELETE statements do not violate RNDS if these statements do not explicitly read any database states, such as columns of a table. However, dynamic INSERT, UPDATE, and DELETE statements always violate RNDS, regardless of whether or not the statements explicitly read database states.

The following INSERT violates RNDS if it is executed dynamically, but it does not violate RNDS if it is executed statically.

INSERT INTO my_table values(3, 'BOB');

The following UPDATE always violates RNDS statically and dynamically, because it explicitly reads the column name of my_table.

UPDATE my_table SET id=777 WHERE name='BOB';

Overloading Packaged PL/SQL Functions

PL/SQL lets you overload packaged (but not standalone) functions; that is, you can use the same name for different functions if their formal parameters differ in number, order, or data type family. However, PRAGMARESTRICT_REFERENCES applies to only one function declaration (the most recently declared one).

In Example 7-21, the pragma applies to the second declaration of valid.

Serially Reusable PL/SQL Packages

PL/SQL packages usually consume user global area (UGA) memory corresponding to the number of package variables and cursors in the package. This limits scalability, because the memory increases linearly with the number of users. The solution is to allow some packages to be marked as SERIALLY_REUSABLE (using pragma syntax).

For serially reusable packages, the package global memory is not kept in the UGA for each user; rather, it is kept in a small pool and reused for different users. Therefore, the global memory for such a package is only used within a unit of work. At the end of that unit of work, the memory can therefore be released to the pool to be reused by another user (after running the initialization code for all the global variables).

The unit of work for serially reusable packages is implicitly a call to the server; for example, an OCI call to the server, or a PL/SQL RPC call from a client to a server, or an RPC call from a server to another server.

Package States

The state of a nonreusable package (one not marked SERIALLY_REUSABLE) persists for the lifetime of a session. A package state includes global variables, cursors, and so on.

The state of a serially reusable package persists only for the lifetime of a call to the server. On a subsequent call to the server, if a reference is made to the serially reusable package, then Oracle Database creates a new instantiation of the serially reusable package and initializes all the global variables to NULL or to the default values provided. Any changes made to the serially reusable package state in the previous calls to the server are not visible.

Note:

Creating a new instantiation of a serially reusable package on a call to the server does not necessarily imply that Oracle Database allocates memory or configures the instantiation object. Oracle Database looks for an available instantiation work area (which is allocated and configured) for this package in a least-recently used (LRU) pool in the SGA.

At the end of the call to the server, this work area is returned back to the LRU pool. The reason for keeping the pool in the SGA is that the work area can be reused across users who have requests for the same package.

Why Serially Reusable Packages?

Because the state of a nonreusable package persists for the lifetime of the session, this locks up UGA memory for the whole session. In some applications, such as Oracle Office, a log-on session typically exists for days. Applications often need certain packages only for short periods of the session. Ideally, such applications could de-instantiate the package state in after they finish using the package (the middle of the session).

SERIALLY_REUSABLE packages enable you to design applications that manage memory better for scalability. Package states that matter only for the duration of a call to the server can be captured in SERIALLY_REUSABLE packages.

Syntax of Serially Reusable Packages

A package can be marked serially reusable by a pragma. The syntax of the pragma is:

PRAGMA SERIALLY_REUSABLE;

A package specification can be marked serially reusable, whether or not it has a corresponding package body. If the package has a body, then the body must have the serially reusable pragma, if its corresponding specification has the pragma; it cannot have the serially reusable pragma unless the specification also has the pragma.

Semantics of Serially Reusable Packages

A package that is marked SERIALLY_REUSABLE has the following properties:

Its package variables are meant for use only within the work boundaries, which correspond to calls to the server (either OCI call boundaries or PL/SQL RPC calls to the server).

Note:

If the application programmer makes a mistake and depends on a package variable that is set in a previous unit of work, then the application program can fail. PL/SQL cannot check for such cases.

A pool of package instantiations is kept, and whenever a "unit of work" needs this package, one of the instantiations is "reused", as follows:

The package variables are reinitialized (for example, if the package variables have default values, then those values are reinitialized).

The initialization code in the package body is run again.

At the "end work" boundary, cleanup is done.

If any cursors were left open, then they are silently closed.

Some nonreusable secondary memory is freed (such as memory for collection variables or long VARCHAR2s).

This package instantiation is returned back to the pool of reusable instantiations kept for this package.

Serially reusable packages cannot be accessed from database triggers or other PL/SQL subprograms that are invoked from SQL statements. If you try, then Oracle Database generates an error.

Examples of Serially Reusable Packages

The two packages specified in Example 7-22 are the same, except that one is serially reusable and the other is not. Neither package has a body.

Open cursors in serially reusable packages are closed automatically at the end of a server call, and must be re-opened in a new server call. A server call can be different from a subprogram call, as Example 7-24 shows.

Returning Large Amounts of Data from a Function

In a data warehousing environment, you might use PL/SQL functions to transform large amounts of data. Perhaps the data is passed through a series of transformations, each performed by a different function. PL/SQL table functions let you perform such transformations without significant memory overhead or the need to store the data in tables between each transformation stage. These functions can accept and return multiple rows, can return rows as they are ready rather than all at once, and can be parallelized.

Coding Your Own Aggregate Functions

To analyze a set of rows and compute a result value, you can code your own aggregate function that works the same as a built-in aggregate like SUM:

Define a SQL object type that defines these member functions:

ODCIAggregateInitialize

ODCIAggregateIterate

ODCIAggregateMerge

ODCIAggregateTerminate

Code the member functions. In particular, ODCIAggregateIterate accumulates the result as it is invoked once for each row that is processed. Store any intermediate results using the attributes of the object type.

Create the aggregate function, and associate it with the new object type.

Call the aggregate function from SQL queries, DML statements, or other places that you might use the built-in aggregates. You can include typical options such as DISTINCT and ALL in the invocation of the aggregate function.